1. Field of the Invention
The present invention relates to a laminated metal gasket used as a cylinder head gasket or the like and interposed between the cylinder head and the cylinder block of an internal combustion engine to seal the surfaces between them.
2. Prior Art
Conventionally, a laminated metal gasket having a sealing structure shown in FIG. 9 around hole portions 100 to be sealed, such as combustion chamber holes, water holes or oil holes provided in a cylinder head gasket is well known. The laminated metal gasket has a structure wherein on both sides of an intermediate plate 103 made of metal and having a folded portion 104 at a peripheral edge portion of the hole portion 100 to be sealed, surface plates 101, 102 are laminated which are made of elastic metal plates and have beads 105, 106 surrounding, outside the folded portion 104, the above-mentioned hole portion 100.
The beads 105 and 106 of the above-mentioned surface plates 101,102 are formed in the same shape having the same dimensions in height and width. The vertexes of the beads 105 and 106 face each other and come into contact with the intermediate plate 103. On the surfaces of the surface plates 101, 102, a thin layer 107 made of sealing material, such as rubber or plastic,is coated to compensate for the roughness, distortion, scratches, etc. on the joint surfaces between which the gasket is sandwiched.
In the above-mentioned sealing structure, the hole portion 100 is double-sealed by the thick gasket portion formed by the folded portion 104 at the peripheral edge portion of the hole portion 100 to be sealed and the beads 105, 106 surrounding the hole portion 100 outside the thick gasket portion, thereby ensuring proper sealing performance.
However, when such joint surfaces as those of the cylinder head and the cylinder block of an engine, for example, are distorted and deformed significantly because of bolt tightening or exposure to high temperature and combustion gas pressure and when the amounts of the distortion and deformation on one joint surface differ considerably from those on the other joint surface, leakage is apt to occur on the joint surface which has been distorted and deformed more significantly in the case of the above-mentioned sealing structure. More particularly, when the cylinder head is made of an aluminum alloy and the cylinder block is made of cast iron, the amounts of the distortion and deformation on one joint surface differ considerably from those of the other surface because of differences in thermal expansion and rigidity. Even in the case when both the cylinder head and the cylinder block are made of an aluminum alloy, the amounts of the distortion and deformation on one joint surface differ from those of the other surface because of difference in rigidity caused by difference in the structures of the two components. Larger distortion and deformation are usually caused on the joint surface of the cylinder head.
In the above-mentioned conventional sealing structure, it is difficult for the inner sealing portion thereof, that is, the thick gasket portion at the peripheral edge portion of the hole portion 100 to be sealed, to follow such significant distortion and deformation. The outer sealing portions, that is, the beads 105, 106 surrounding the hole portion 100 follow the distortion and deformation on the joint surfaces by elastic deformation to seal the hole portion 100. However, when the distortion and deformation on the joint surfaces are significant and when the amounts of the distortion and deformation on one joint surface differ considerably from those on the other joint surface, the amount of restoration of the bead of the surface plate coming into contact with the joint surface which-has been distorted more significantly becomes insufficient. As a result, necessary sealing pressure cannot be obtained, and leakage is caused.